This just literally worked for me after over a month, 4 visits to ATT, one to UBreakit, and countless hours of tech support. I was trying a few things on my own and one of these things or a combination immediately reactivated my 4G LTE. It originated with me being curious if there was an issue with the bands. #1, have them check your plan or check yourself if you can. My plan was listed as 4G LTE (No Visual Voicemail) I changed it online to the regular 4G LTE plan. #2, Go do your phone keypad and enter this: *#197328640# A screen should load immediately. Select BASIC INFO. look for dBm readings. if no signals, hit the menu at the top right and select BACK. When you reach the main menu, select WB AMR, select SETTING, UMTS ENABLE (*) and GSM DISABLE(*). Don't know if it matters, but I did it. Exit this menu via home button #3 Go to your keypad again. Enter: *#2663# . At the top you should see a button that says TSP FW Update (General) select it and let the phone update. (NOTE: you may think you've already updated your phone to the latest version, but software is different from firmware (FW), and no one suggested this in any of my ATT or Samsung troubleshooting so it's worth a try. I returned to my home screen, turned off WIFI to test, 4G LTE came up immediately. I really truly hope this helps anyone with this ridiculously awful problem that no amount of tech support seems capable of working out. For any techies still troubleshooting, here's a link of those Samsung access codes to explore on your own http://www.samsungsfour.com/tutorials/all-secret-codes-of-samsung-galaxy-s10-galaxy-s10-and-s10e.html.
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